This invention relates generally to seismic exploration and more particularly to a system for removing a substantially sinusoidal interfering signal from a seismic signal made up of the interfering signal and of signals produced as a result of seismic energy detected during the course of seismic exploration.
The problem of eliminating from recordings of geophone signals certain monochromatic interfering signals that are picked up by the long leads that are customarily used for connecting geophones to recorders is not a new one. Various attempts have been made to eliminate such interfering signals, which are usually produced by 60-cycle or 120-cycle electromagnetic energy originating from power lines. For example, notch filters have been used for many years for this purpose. A difficulty with the use of notch filters is that they produce considerable phase distortion for signals having frequencies in the vicinity of the frequency to be eliminated by the filter. Also, attempts have been made to produce nulling signals of opposite time-phase relationship from the detected monochromatic signals to be eliminated as evidenced by the following U.S. Pat. Nos. 2,438,217-Johnson; 2,733,412-Alexander; 3,757,235-Tvedt et al. Apparatus making use of such a technique has generally not been satisfactory in the past because the frequency and phase of the interfering signal is not stable and any variations thereof will produce a residual signal on the seismic records. The nulling signal has to match the interfering signal in amplitude, frequency and phase if it is to satisfactorily cancel the interfering signal. Systems used in the past have been deficient in this respect, particularly in view of the fact that the frequency, the phase, and the amplitude of the interfering signal are subject to change from time to time.